1. Field of the Invention
The present invention relates to a heat treatment apparatus for accurately treating a workpiece and a method thereof.
2. Description of the Related Art
When a diffusion layer, a silicon oxide film, a silicon nitride film, or the like is formed on a workpiece (such as a semiconductor wafer or a glass substrate on which an amorphous Si layer is formed for an LCD), various heat treatment apparatuses are used. In these apparatuses, a workpiece is placed in a process tube that is a reaction vessel. In the process tube, the workpiece is heated. In addition, an inert gas or a carrier gas (which is referred to as a process gas) is supplied to the process tube so as to perform a predetermined heat treatment.
In such a heat treatment apparatus, after the treatment is completed, the inside of the process tube is cooled to a predetermined temperature. Thereafter, the workpiece is unloaded from the process tube. However, if a hot workpiece were unloaded to the outside of the apparatus, an oxide film would grow on the surface of the workpiece. Thus, the yield of the workpiece would decrease and the characteristics of the workpiece, in particular, the characteristics of the semiconductor devices would degrade. As high speed operations and high integration have been required for integrated circuits, heat diffusion of impurities should have been performed in shallow regions. Thus, technologies for improving diffusing accuracies and controlling the depth of diffusion of impurities with good repeatability have been required. To control the process of the shallow diffusion, a semiconductor wafer, which is a workpiece, should be heated to a diffusion process temperature in a short time. In addition, the diffusion process temperature holding time and the temperature drop time should be repeated by the same program.
Technologies for quickly cooling the process tube to a predetermined temperature and equally cooling the entire surface of each wafer are disclosed as Japanese Utility Model Registration Laid-Open Publication Nos. 63-121429 and 63-8128.
According to the former publication, a process tube is cooled by a spiral air current formed in the longitudinal direction thereof. According to the latter publication, at least one air blow pipe for supplying cooling compression air is disposed between a heating coil and a process tube. Air is exhausted from an opposite furnace opening.
However, in the constructions described in these publications, the cooling speed and cooling equality were not sufficient.
In other words, according to the construction of the former publication, it was difficult to generate the spiral air current along the process tube. Even if the spiral air current were generated, a guide means would be required. Thus, the construction would be complicated. In addition, since the air current would be subject to flow resistance, sufficient cooling speed could not be obtained.
According to the construction of the latter publication, since cooling air is supplied and exhausted by pipes, air current would be irregularly generated. In addition, when air is supplied and drained by the pipes, the flow amount and flow speed of the air tend to be restricted. Thus, the improvement of the cooling equality is limited. Moreover, in this construction, although the cooling air is forcedly supplied, the waste air is not forcedly exhausted. Thus, it is difficult to generate an equal air current.
In recent years, high speed heat treatment apparatuses with a temperature rise/drop speed of at least 30.degree. C./min or higher, preferably 100.degree. C./min (temperature rise state) and 60.degree. C./min (temperature drop state) have been required. Thus, a growth of for example an oxide film in loading/unloading temperature setting periods rather than real treatment process adversely affects fabrications of highly integrated memory chips of 16 MB and 64 MB. On the other hand, the process tube is heated by a heat generating resistor. The heat generating resistor should equally heat the process tube. In addition, the heat generating resistor should have a simple construction and prevent an interference with a temperature detecting portion.